1. Field of the Invention
Embodiments disclosed herein relate generally to cutting elements for drilling earth formations. More specifically, embodiments disclosed herein relate to cutting elements containing metal carbide substrates.
2. Background Art
In a typical drilling operation, a drill bit is rotated while being advanced into a soil or rock formation. The formation is cut by cutting elements on the drill bit, and the cuttings are flushed from the borehole by the circulation of drilling fluid that is pumped down through the drill string and flows back toward the top of the borehole in the annulus between the drill string and the borehole wall. The drilling fluid is delivered to the drill bit through a passage in the drill stem and is ejected outwardly through nozzles in the cutting face of the drill bit. The ejected drilling fluid is directed outwardly through the nozzles at high speed to aid in cutting, flush the cuttings and cool the cutter elements.
There are several types of drill bits, including roller cone bits, hammer bits and drag bits. Roller cone rock bits include a bit body adapted to be coupled to a rotatable drill string and include at least one “cone” that is rotatably mounted to a cantilevered shaft or journal as frequently referred to in the art. Each roller cone in turn supports a plurality of cutting elements that cut and/or crush the wall or floor of the borehole and thus advance the bit. The cutting elements, either inserts or milled teeth, contact with the formation during drilling. Hammer bits typically include a one piece body with having crown. The crown includes inserts pressed therein for being cyclically “hammered” and rotated against the earth formation being drilled.
Drag bits, often referred to as “fixed cutter drill bits,” include bits that have cutting elements attached to the bit body, which may be a steel bit body or a matrix bit body formed from a matrix material such as tungsten carbide surrounded by a binder material. Drag bits may generally be defined as bits that have no moving parts. However, there are different types and methods of forming drag bits that are known in the art. For example, drag bits having abrasive material, such as diamond, impregnated into the surface of the material which forms the bit body are commonly referred to as “impreg” bits. Drag bits having cutting elements made of an ultra hard cutting surface layer or “table” (typically made of polycrystalline diamond material or polycrystalline boron nitride material) deposited onto or otherwise bonded to a substrate are known in the art as polycrystalline diamond compact (“PDC”) bits, or more broadly as shear cutter bits. Shear cutter bits drill soft formations easily, but they are frequently used to drill moderately hard or abrasive formations. They cut rock formations with a shearing action using small cutting elements referred to as shear cutters that do not penetrate deeply into the formation. Because the penetration depth is shallow, high rates of penetration are achieved through relatively high bit rotational velocities.
An example of a shear cutter bit is shown in FIG. 1. FIG. 1 shows a rotary drill bit 10 includes a bit body 12 having a cutting end 11 and a threaded pin end 13 for connection to a drill string (not shown). The cutting end 11 of the bit body 12 is formed with a plurality of blades 14, which extend generally outwardly away from a central longitudinal axis of rotation 16 of the drill bit. A plurality of shear cutters 18 having a cutting layer 19 bonded to a carbide substrate 17 are disposed side by side along the length of each blade. The number of shear cutters 18 carried by each blade may vary. The shear cutters are positioned along the leading edges of the bit body blades so that as the bit body is rotated, the shear cutters engage and drill the earth formation. In use, high forces may be exerted on the shear cutters, particularly in the forward-to-rear direction. Additionally, the bit and the shear cutters may be subjected to substantial abrasive forces. In some instances, impact, vibration and erosive forces have caused drill bit failure due to loss of one or more cutters, or due to breakage of the blades.
In a typical shear cutter, a compact of polycrystalline diamond (“PCD”) (or other superhard material, such as polycrystalline cubic boron nitride) is bonded to a substrate material, which is typically a sintered metal-carbide, to form a cutting structure. A PCD shear cutter may be formed by placing a mixture of diamond grains or diamond grains and catalyst material on a substrate and subjecting the assembly to high pressure, high temperature (“HPHT”) conditions. Alternatively, a pre-formed diamond table may be placed on a substrate and subjected to HPHT conditions to bond the diamond table to the substrate. During the HPHT process, metal binder migrates from the substrate and passes through the diamond grains to promote intercrystalline growth between the diamond grains, binding the diamond grains to each other and binding the formed PCD table to the substrate. In particular, PCD refers to a polycrystalline mass of diamond grains or crystals that are bonded together to form an integral, tough, high-strength mass or lattice. The resulting PCD structure produces enhanced properties of wear resistance and hardness, making PCD materials extremely useful in aggressive wear and cutting applications where high levels of wear resistance and hardness are desired.
Shear cutter substrates are commonly formed from a carbide/metal composite (often referred to as a cermet), which includes hard particles of carbide surrounded by a metal binder, typically cobalt, which acts as a matrix. The individual hard particles thus are embedded in a matrix of a relatively ductile metal such that the ductile metal matrix provides the necessary toughness, while the grains of hard material in the matrix furnish the necessary wear resistance. The ductile metal matrix also reduces crack formation and suppresses crack propagation through the composite material once a crack has been initiated.
Due to its toughness and high wear resistance, cemented tungsten carbide is a common cermet that is used to form cutting element substrates in rock-drilling and earth boring applications. “Cemented tungsten carbide” generally refers to a tungsten carbide composite which comprises tungsten carbide (“WC”) grains bonded together by a binder phase. Among the types of tungsten carbide particles that may be used to form a cemented tungsten carbide, for example, include cast tungsten carbide, macro-crystalline tungsten carbide, carburized tungsten carbide and cemented tungsten carbide. In most applications, the binder phase comprises cobalt (Co), nickel (Ni), and/or iron (Fe). However, tungsten carbide grains dispersed in a cobalt binder matrix is the most common form of cemented tungsten carbide currently used for cutting elements in drilling applications, and is typically classified by grades based on the grain size of the tungsten carbide particles used and the cobalt content. However, in some cases, cemented tungsten carbide may be classified by grades based on the cobalt content and a material property such as hardness or wear resistance.
FIG. 2 illustrates the conventional microstructure of a tungsten carbide/metal composite. As shown in FIG. 2, cemented tungsten carbide 20 includes tungsten carbide grains 22 that are bonded to one another by a metal binder phase 24. As illustrated, tungsten carbide grains may be bonded to other grains of tungsten carbide (depending on the metal content), thereby having a tungsten carbide/tungsten carbide interface 26, and/or may be bonded to the metal phase, thereby having a tungsten carbide/metal interface 25. The unique properties of tungsten carbide cermets result from this combination of hard carbide particles with a tougher, ductile metal phase.
In conventional carbide cermets, it is possible to increase the toughness of the composite by increasing the amount of metal binder present in the composite and/or by increasing the carbide grain size. Conversely, the hardness of the carbide cermet may be increased by decreasing the amount of metal binder and/or by decreasing the carbide grain size. Thus, toughness and hardness are inversely related. To utilize both characteristics of toughness and hardness, some prior art cermets have been designed to have areas with higher amounts of binder (increased toughness) and areas with lower amounts of binder (increased hardness) by forming a binder gradient.
For example, U.S. Pat. Nos. 7,699,904 and 7,569,179, which are incorporated herein by reference, describe methods of forming functionally graded materials having a metal matrix phase, such as cobalt, and a hard phase made of at least two chemical elements, such as tungsten and carbon. The functionally graded composites have a continuous gradient of the metal matrix phase that is formed by designing an initial (non-continuous) gradient of one of the chemical elements of the hard phase and then liquid phase sintering the hard phase and metal matrix phase. For example, an initial gradient for tungsten carbide may be formed by creating a first layer deficient in carbon and a second layer enriched with carbon. When the tungsten carbide layers are sintered with the metal matrix phase, the heated conditions cause the carbon atoms to diffuse in a direction from the enriched layer to the deficient layer and atoms of the metal matrix to flow in the same direction as the diffusion.
Other prior art methods of forming continuous gradient of the matrix metal phase may include, for example, creating a graded structure by using two layers with different magnetic saturation numbers, as described in U.S. Pat. No. 5,541,006, and creating a graded structure through a carburizing treatment, as described in U.S. Pat. No. 6,896,460. However, such methods have limitations with respect to the size of gradient that may be formed. In particular, gradients formed using different magnetic saturation may be limited to a metal matrix gradient having only 1-2% difference, and gradients formed by carburization treatments may be limited to small depths of the gradient, as measured from the surface of the treated composite. Also, this process requires formation of an eta (η) phase (i.e., a complex carbide compound of tungsten, cobalt, and carbon), which has been known in the art as forming brittle grains around WC crystals, and thus, sites for crack initiation and propagation. Thus, this prior art gradient-forming method requires forming a hard phase element deficient layer and a hard phase element enriched layer in order to create a continuous gradient of the matrix metal phase.
Moreover, it has not yet been known to use graded carbides such as the ones described above in a shear cutter substrate, which undergoes HPHT processing to attach an ultra-hard cutting layer to the substrate. Accordingly, there is a need for improved cutting element substrates that have properties of both increased toughness and increased hardness and that may be bonded to an ultra-hard cutting table.